Method of addressing messages and communications

ABSTRACT

A method of establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising utilizing a tree search method to attempt to identify individual ones of the multiple wireless identification devices so as to be able to perform communications, without collision, between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple nodes respectively representing subgroups of the multiple wireless identification devices, wherein the interrogator transmits a command at a node, requesting that devices within the subgroup represented by the node respond, wherein the interrogator determines if a collision occurs in response to the command and, if not, repeats the command at the same node. An interrogator configured to transmit a command at a node, requesting that devices within the subgroup represented by the node respond, the interrogator further being configured to determine if a collision occurs in response to the command and, if not, to repeat the command at the same node . In one aspect, a method implemented in an RFID device includes: receiving a first command, comprising a first set of bit values, from an interrogator to select the RFID device based on a comparison between the first set of bit values and data stored in the RFID device; transmitting a response to the first command, the response including an identifier of the RFID device; if the response is received at an interrogator, remaining silent when the first command is subsequently repeated; and if the response is not received at the interrogator, re- transmitting the response when the first command is subsequently repeated.

CROSS REFERENCE TO RELATED APPLICATION

ThisMore than one reissue application has been filed for the reissue ofU.S. Pat. No. 6,282,186, which The reissue applications are the initialreissue application Ser. No. 10/652,573 filed Aug. 28, 2003, acontinuation reissue application Ser. No. 11/862,121, filed Sep. 26,2007, a continuation reissue application Ser. No. 11/862,130, filed Sep.26, 2007, and the present continuation reissue application, which is aContinuationcontinuation application of a reissue application Ser. No.10/652,573, filed Aug. 28, 2003, which is a reissue application of U.S.patent application Ser. No. 09/556,235, now U.S. Pat. No. 6,282,186,which is a continuation application of U.S. patent application Ser. No.09/026,050, filed Feb. 19, 1998, now U.S. Pat. No. 6,061,344 and titled“Method of Addressing Messages and Communications System”.

TECHNICAL FIELD

This invention relates to communications protocols and to digital datacommunications. Still more particularly, the invention relates to datacommunications protocols in mediums such as radio communication or thelike. The invention also relates to radio frequency identificationdevices for inventory control, object monitoring, determining theexistence, location or movement of objects, or for remote automatedpayment.

BACKGROUND OF THE INVENTION

Communications protocols are used in various applications. For example,communications protocols can be used in electronic identificationsystems. As large numbers of objects are moved in inventory, productmanufacturing, and merchandising operations, there is a continuouschallenge to accurately monitor the location and flow of objects.Additionally, there is a continuing goal to interrogate the location ofobjects in an inexpensive and streamlined manner. One way of trackingobjects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another wireless electronic identification system utilizes a large,board level, active transponder device affixed to an object to bemonitored which receives a signal from an interrogator. The devicereceives the signal, then generates and transmits a responsive signal.The interrogation signal and the responsive signal are typicallyradio-frequency (RF) signals produced by an RF transmitter circuit.Because active devices have their own power sources, and do not need tobe in close proximity to an interrogator or reader to receive power viamagnetic coupling. Therefore, active transponder devices tend to be moresuitable for applications requiring tracking of a tagged device that maynot be in close proximity to an interrogator. For example, activetransponder devices tend to be more suitable for inventory control ortracking.

Electronic identification systems can also be used for remote payment.For example, when a radio frequency identification device passes aninterrogator at a toll booth, the toll booth can determine the identityof the radio frequency identification device, and thus of the owner ofthe device, and debit an account held by the owner for payment of tollor can receive a credit card number against which the toll can becharged. Similarly, remote payment is possible for a variety of othergoods or services.

A communication system typically includes two transponders: a commanderstation or interrogator, and a responder station or transponder devicewhich replies to the interrogator.

If the interrogator has prior knowledge of the identification number ofa device which the interrogator is looking for, it can specify that aresponse is requested only from the device with that identificationnumber. Sometimes, such information is not available. For example, thereare occasions where the interrogator is attempting to determine which ofmultiple devices are within communication range.

When the interrogator sends a message to a transponder device requestinga reply, there is a possibility that multiple transponder devices willattempt to respond simultaneously, causing a collision, and thus causingan erroneous message to be received by the interrogator. For example, ifthe interrogator sends out a command requesting that all devices withina communications range identify themselves, and gets a large number ofsimultaneous replies, the interrogator may not be able to interpret anyof these replies. Thus, arbitration schemes are employed to permitcommunications free of collisions.

In one arbitration scheme or system, described in commonly assigned U.S.Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all toSnodgrass et al. and all incorporated herein by reference, theinterrogator sends a command causing each device of a potentially largenumber of responding devices to select a random number from a knownrange and use it as that device's arbitration number. By transmittingrequests for identification to various subsets of the full range ofarbitration numbers, and checking for an error-free response, theinterrogator determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator is able to conduct subsequent uninterrupted communicationwith devices, one at a time, by addressing only one device.

Another arbitration scheme is referred to as the Aloha or slotted Alohascheme. This scheme is discussed in various references relating tocommunications, such as Digital Communications: Fundamentals andApplication, Bernard Sklar, published January 1988 by Prentice Hall. Inthis type of scheme, a device will respond to an interrogator using oneof many time domain slots selected randomly by the device. A problemwith the Aloha scheme is that if there are many devices, or potentiallymany devices in the field (i.e. in communications range, capable ofresponding) then there must be many available slots or many collisionswill occur. Having many available slots slows down replies. If themagnitude of the number of devices in a field is unknown, then manyslots are needed. This results in the system slowing down significantlybecause the reply time equals the number of slots multiplied by the timeperiod required for one reply.

An electronic identification system which can be used as a radiofrequency identification device, arbitration schemes, and variousapplications for such devices are described in detail in commonlyassigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29,1996 and now U.S. Pat. No. 6,130,602, and incorporated herein byreference.

SUMMARY OF THE INVENTION

The invention provides a wireless identification device configured toprovide a signal to identify the device in response to an interrogationsignal.

One aspect of the invention provides a method implemented in an RFIDdevice. The method includes: receiving a first command, comprising afirst set of bit values, from an interrogator to select the RFID devicebased on a comparison between the first set of bit values and datastored in the RFID device, transmitting a response to the first command,the response including an identifier of the RFID device; if the responseis received at an interrogator, remaining silent when the first commandis subsequently repeated; and if the response is not received at theinterrogator, re-transmitting the response when the first command issubsequently repeated.

One aspect of the invention provides a method of establishing wirelesscommunications between an interrogator and individual ones of multiplewireless identification devices. The method comprises utilizing a treesearch method to attempt to identify individual ones of the multiplewireless identification devices so as to be able to performcommunications, without collision, between the interrogator andindividual ones of the multiple wireless identification devices. Asearch tree is defined for the tree search method. The tree has multiplenodes respectively representing subgroups of the multiple wirelessidentification devices. The interrogator transmits a command at a node,requesting that devices within the subgroup represented by the noderespond. The interrogator determines if a collision occurs in responseto the command and, if not, repeats the command at the same node.

Another aspect of the invention provides a communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion. The interrogator is configured to employ tree searching toattempt to identify individual ones of the multiple wirelessidentification devices, so as to be able to perform communicationswithout collision, between the interrogator and individual ones of themultiple wireless identification devices. The interrogator is configuredto follow a search tree, the tree having multiple nodes respectivelyrepresenting subgroups of the multiple wireless identification devices.The interrogator is configured to transmit a command at a node,requesting that devices within the subgroup represented by the noderespond. The interrogator is further configured to determine if acollision occurs in response to the command and, if not, to repeat thecommand at the same node.

One aspect of the invention provides a radio frequency identificationdevice comprising an integrated circuit including a receiver, atransmitter, and a microprocessor. In one embodiment, the integratedcircuit is a monolithic single die single metal layer integrated circuitincluding the receiver, the transmitter, and the microprocessor. Thedevice of this embodiment includes an active transponder, instead of atransponder which relies on magnetic coupling for power and thereforehas a much greater range.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a high level circuit schematic showing an interrogator and aradio frequency identification device embodying the invention.

FIG. 2 is a front view of a housing, in the form of a badge or card,supporting the circuit of FIG. 1 according to one embodiment theinvention.

FIG. 3 is a front view of a housing supporting the circuit of FIG. 1according to another embodiment of the invention.

FIG. 4 is a diagram illustrating a tree splitting sort method forestablishing communication with a radio frequency identification devicein a field of a plurality of such devices.

FIG. 5. is a diagram illustrating a modified tree splitting sort methodfor establishing communication with a radio frequency identificationdevice in a field of a plurality of such devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless identification device 12 in accordancewith one embodiment of the invention. In the illustrated embodiment, thewireless identification device is a radio frequency data communicationdevice 12, and includes RFID circuitry 16. The device 12 furtherincludes at least one antenna 14 connected to the circuitry 16 forwireless or radio frequency transmission and reception by the circuitry16. In the illustrated embodiment, the RFID circuitry is defined by anintegrated circuit as described in the above-incorporated patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996 and now U.S. Pat.No. 6,130,602. Other embodiments are possible. A power source or supply18 is connected to the integrated circuit 16 to supply power to theintegrated circuit 16. In one embodiment, the power source 18 comprisesa battery.

The device 12 transmits and receives radio frequency communications toand from an interrogator 26. An exemplary interrogator is described incommonly assigned U.S. patent application Ser. No. 08/907,689, filedAug. 8, 1997 and now U.S. Pat. No. 6,289,209, which is incorporatedherein by reference. Preferably, the interrogator 26 includes an antenna28, as well as dedicated transmitting and receiving circuitry, similarto that implemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal orcommand 27 via the antenna 28. The device 12 receives the incominginterrogation signal via its antenna 14. Upon receiving the signal 27,the device 12 responds by generating and transmitting a responsivesignal or reply 29. The responsive signal 29 typically includesinformation that uniquely identifies, or labels the particular device 12that is transmitting, so as to identify any object or person with whichthe device 12 is associated. Although only one device 12 is shown inFIG. 1, typically there will be multiple devices 12 that correspond withthe interrogator 26, and the particular devices 12 that are incommunication with the interrogator 26 will typically change over time.In the illustrated embodiment in FIG. 1, there is no communicationbetween multiple devices 12. Instead, the devices 12 respectivelycommunicate with the interrogator 26. Multiple devices 12 can be used inthe same field of an interrogator 26 (i.e., within communications rangeof an interrogator 26).

The radio frequency data communication device 12 can be included in anyappropriate housing or packaging. Various methods of manufacturinghousings are described in commonly assigned U.S. patent application Ser.No. 08/800,037, filed Feb. 13, 1997, and now U.S. Pat. No. 5,988,510,which is incorporated herein by reference.

FIG. 2 shows but one embodiment in the form of a card or badge 19including a housing 11 of plastic or other suitable material supportingthe device 12 and the power supply 18. In one embodiment, the front faceof the badge has visual identification features such as graphics, text,information found on identification or credit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 shows a miniature housing 20 encasing thedevice 12 and power supply 18 to define a tag which can be supported byan object (e.g., hung from an object, affixed to an object, etc.).Although two particular types of housings have been disclosed, otherforms of housings are employed in alternative embodiments.

If the power supply 18 is a battery, the battery can take any suitableform. Preferably, the battery type will be selected depending on weight,size, and life requirements for a particular application. In oneembodiment, the battery 18 is a thin profile button-type cell forming asmall, thin energy cell more commonly utilized in watches and smallelectronic devices requiring a thin profile. A conventional button-typecell has a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the powersource 18 comprises a series connected pair of button type cells. Inother alternative embodiments, other types of suitable power source areemployed.

The circuitry 16 further includes a backscatter transmitter and isconfigured to provide a responsive signal to the interrogator 26 byradio frequency. More particularly, the circuitry 16 includes atransmitter, a receiver, and memory such as is described in U.S. patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996 and now U.S. Pat.No. 6,130,602.

Radio frequency identification has emerged as a viable and affordablealternative to tagging or labeling small to large quantities of items.The interrogator 26 communicates with the devices 12 via anelectromagnetic link, such as via an RF link (e.g., at microwavefrequencies, in one embodiment), so all transmissions by theinterrogator 26 are heard simultaneously by all devices 12 within range.

If the interrogator 26 sends out a command requesting that all devices12 within range identify themselves, and gets a large number ofsimultaneous replies, the interrogator 26 may not be able to interpretany of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification numberof a device 12 which the interrogator 26 is looking for, it can specifythat a response is requested only from the device 12 with thatidentification number. To target a command at a specific device 12,(i.e., to initiate point-on-point communication), the interrogator 26must send a number identifying a specific device 12 along with thecommand. At start-up, or in a new or changing environment, theseidentification numbers are not known by the interrogator 26. Therefore,the interrogator 26 must identify all devices 12 in the field (withincommunication range) such as by determining the identification numbersof the devices 12 in the field. After this is accomplished,point-to-point communication can proceed as desired by the interrogator26.

Generally speaking, RFID systems are a type of multiaccess communicationsystem. The distance between the interrogator 26 and devices 12 withinthe field is typically fairly short (e.g., several meters), so packettransmission time is determined primarily by packet size and baud rate.Propagation delays are negligible. In such systems, there is a potentialfor a large number of transmitting devices 12 and there is a need forthe interrogator 26 to work in a changing environment, where differentdevices 12 are swapped in and out frequently (e.g., as inventory isadded or removed). In such systems, the inventors have determined thatthe use of random access methods work effectively for contentionresolution (i.e., for dealing with collisions between devices 12attempting to respond to the interrogator 26 at the same time).

RFID systems have some characteristics that are different from othercommunications systems. For example, one characteristic of theillustrated RFID systems is that the devices 12 never communicatewithout being prompted by the interrogator 26. This is in contrast totypical multiaccess systems where the transmitting units operate moreindependently. In addition, contention for the communication medium isshort lived as compared to the ongoing nature of the problem in othermultiaccess systems. For example, in a RFID system, after the devices 12have been identified, the interrogator can communicate with them in apoint-to-point fashion. Thus, arbitration in a RFID system is atransient rather than steady-state phenomenon. Further, the capabilityof a device 12 is limited by practical restrictions on size, power, andcost. The lifetime of a device 12 can often be measured in terms ofnumber of transmissions before battery power is lost. Therefore, one ofthe most important measures of system performance in RFID arbitration istotal time required to arbitrate a set of devices 12. Another measure ispower consumed by the devices 12 during the process. This is in contrastto the measures of throughput and packet delay in other types ofmultiaccess systems.

FIG. 4 illustrates one arbitration scheme that can be employed forcommunication between the interrogator and devices 12. Generally, theinterrogator 26 sends a command causing each device 12 of a potentiallylarge number of responding devices 12 to select a random number from aknown range and use it as that device's arbitration number. Bytransmitting requests for identification to various subsets of the fullrange of arbitration numbers, and checking for an error-free response,the interrogator 26 determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator 26 is able to conduct subsequent unterrupted communicationwith devices 12, one at a time, by addressing only one device 12.

Three variables are used: an arbitration value (AVALUE), an arbitrationmask (AMASK), and a random value ID (RV). The interrogator sends anIdentify command (IdentifyCmnd) causing each device of a potentiallylarge number of responding devices to select a random number from aknown range and use it as that device's arbitration number. Theinterrogator sends an arbitration value (AVALUE) and an arbitration mask(AMASK) to a set of devices 12. The receiving devices 12 evaluate thefollowing equation: (AMASK & AVALUE)==(AMASK & RV) wherein “&” is abitwise AND function, and wherein “==” is an equality function. If theequation evaluates to “1” (TRUE), then the device 12 will reply. If theequation evaluates to “0” (FALSE), then the device 12 will not reply. Byperforming this in a structured manner, with the number of bits in thearbitration mask being increased by one each time, eventually a device12 will respond with no collisions. Thus, a binary search treemethodology is employed.

An example using actual numbers will now be provided using only fourbits, for simplicity, reference being made to FIG. 4. In one embodiment,sixteen bits are used for AVALUE and AMASK. Other numbers of bits canalso be employed depending, for example, on the number of devices 12expected to be encountered in a particular application, on desired costpoints, etc.

Assume, for this example, that there are two devices 12 in the field,one with a random value RV) of 1100 (binary), and another with a randomvalue (RV) of 1010 (binary). The interrogator is tying to establishcommunications without collisions being caused by the two devices 12attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or “don't care” for all bits, asindicated by the character “X” in FIG. 4) and AMASK to 0000. Theinterrogator transmits a command to all devices 12 requesting that theyidentify themselves. Each of the devices 12 evaluate (AMASK &AVALUE)==(AMASK & RV) using the random value RV that the respectivedevices 12 selected. If the equation evaluates to “1” (TRUE), then thedevice 12 will reply. If the equation evaluates to “0” (FALSE), then thedevice 12 will not reply. In the first level of the illustrated tree,AMASK is 0000 and anything bitwise ANDed with all zeros results in allzeros, so both the devices 12 in the field respond, and there is acollision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 andtransmits an Identify command. Both devices 12 in the field have a zerofor their least significant bit, and (AMASK & AVALUE)==(AMASK & RV) willbe true for both devices 12. For the device 12 with a random value of1100, the left side of the equation is evaluated as follows (0001 &0000)=0000.

The right side is evaluated as (0001 & 1100)=0000. The left side equalsthe right side, so the equation is true for the device 12 with therandom value of 1100. For the device 12 with a random value of 1010, theleft side of the equation is evaluated as (0001 & 0000)=0000. The rightside is evaluated as (0001 & 1010)=0000. The left side equals the rightside, so the equation is true for the device 12 with the random value of1010. Because the equation is true for both devices 12 in the field,both devices 12 in the field respond, and there is another collision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE stillat 0000 and transmits an Identify command. (AMASK & AVALUE)==(AMASK &RV) is evaluated for both devices 12. For the device 12 with a randomvalue of 1100, the leftside of the equation is evaluated as follows(0011 & 0000)=0000. The right side is evaluated as (0011 & 1100)=0000.The left side equals the right side, so the equation is true for thedevice 12 with the random value of 1100, so this device 12 responds. Forthe device 12 with a random value of 1010, the left side of the equationis evaluated as (0011 & 0000)=0000. The right side is evaluated as (0011& 1010)=0010. The left side does not equal the right side, so theequation is false for the device 12 with the random value of 1010, andthis device 12 does not respond; Therefore, there is no collision, andthe interrogator can determine the identity (e.g., an identificationnumber) for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the sameAMASK level are accessed when AVALUE is set at 0010, and AMASK is set to0011.

The device 12 with the random value of 1010 receives a command andevaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side ofthe equation is evaluated as (0011 & 0010)=0010. The right side of theequation is evaluated as (0011 & 1010)=0010. The right side equals theleft side, so the equation is true for the device 12 with the randomvalue of 1010. Because there are no other devices 12 in the subtree, agood reply is returned by the device 12 with the random value of 1010.There is no collision, and the interrogator 26 can determine theidentity (e.g., an identification number) for the device 12 that doesrespond.

By recursion, what is meant is that a function makes a call to itself.In other words, the function calls itself within the body of thefunction. After the called function returns, de-recursion takes placeand execution continues at the place just after the function call; i.e.at the beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered1,2,3,4) in it, and the second statement is a recursive call. Assumethat the fourth statement is a return statement. The first time throughthe loop (iteration 1) the function executes the statement 2 and(because it is a recursive call) calls itself causing iteration 2 tooccur. When iteration 2 gets to statement 2, it calls itself makingiteration 3. During execution in iteration 3 of statement 1, assume thatthe function does a return. The information that was saved on the stackfrom iteration 2 is loaded and the function resumes execution atstatement 3 (in iteration 2), followed by the execution of statement 4which is also a return statement. Since there are no more statements inthe function, the function de-recurses to iteration 1. Iteration 1, hadpreviously recursively called itself in statement 2. Therefore, it nowexecutes statement 3 (in iteration 1). Following that it executes areturn at statement 4. Recursion is known in the art.

Consider the following code which can be used to implement operation ofthe method shown in FIG. 4 and described above.

Arbitrate(AMASK,AVALUE) { collision=IdentifyCmnd(AMASK, AVALUE) if(collision) then { /* recursive call for left side */ Arbitrate((AMASK<<1)+1, AVALUE /* recursive call for right side */ Arbitrate((AMASK<<1)+1, AVALUE+(AMASK+1)) } /* endif */ }/* return */

The symbol “<<” represents a bitwise left shift. “<<1” means shift leftby one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK isoriginally called with a value of zero, and 0000<<1 is still 0000.Therefore, for the first recursive call, AMASK=(AMASK<<1)+1. So for thefirst recursive call, the value of AMASK is 0000+0001=0001. For thesecond call, AMASK=(0001<<)+1=0010+1=0011. For the third recursive call,AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by theinterrogator in an Identify command “IdentifyCmnd.” Note that theroutine calls itself if there is a collision. De-recursion occurs whenthere is no collision. AVALUE and AMASK would have values such as thefollowing assuming collisions take place all the way down to the bottomof the tree.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 1000 1111* 0100 0111 0100  1111* 1100  1111*

This sequence of AMASK, AVALUE binary numbers assumes that there arecollisions all the way down to the bottom of the tree, at which pointthe Identify command sent by the interrogator is finally successful sothat no collision occurs. Rows in the table for which the interrogatoris successful in receiving a reply without collision are marked with thesymbol “*”. Note that if the Identify command was successful at, forexample, the third line in the table then the interrogator would stopgoing down that branch of the tree and start down another, so thesequence would be as shown in the following table.

AVALUE AMASK 0000 0000 0000 0001 0000  0011* 0010 0011 . . . . . .

This method is referred to as a splitting method. It works by splittinggroups of colliding devices 12 into subsets that are resolved in turn.The splitting method can also be viewed as a type of tree search. Eachsplit moves the method one level deeper in the tree. Either depth-firstor breadth-first traversals of the tree can be employed. Depth firsttraversals are performed by using recursion, as is employed in the codelisted above. Breadth-first traversals are accomplished by using a queueinstead of recursion.

Either depth-first or breadth-first traversals of the tree can beemployed. Depth first traversals are performed by using recursion, as isemployed in the code listed above. Breadth-first traversals areaccomplished by using a queue instead of recursion. The following is anexample of code for performing a breadth-first traversal.

Arbitrate(AMASK,AVALUE) { enqueue(0,0) while (queue != empty)(AMASK,AVALTE)=dequeue( ) collision=IdentifyCmnd(AMASK, AVALUE) if(collision) then { TEMP = AMASK+1 NEW_AMASK = (AMASK<<1)+1enqueue(NEW_AMASK, AVALUE) enqueue(NEW_AMASK, AVALUE+TEMP) } /* endif */endwhile }/* return */

The symbol “!=” means not equal to. AVALUE and AMASK would have valuessuch as those indicated in the following table for such code.

AVALUE AMASK 0000 0000 0000 0001 0001 0001 0000 0011 0010 0011 0001 00110011 0011 0000 0111 0100 0111 . . . . . .

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

FIG. 5 illustrates an embodiment wherein the interrogator 26 retries onthe same node that yielded a good reply. The search tree has a pluralityof nodes 51, 52, 53, 54 etc. at respective levels 32, 34, 36, 38, or 40.The size of subgroups of random values decrease in size by half witheach node descended.

The interrogator performs a tree search, either depth-first orbreadth-first in a manner such as that described in connection with FIG.4, except that if the interrogator determines that no collision occurredin response to an Identify command, the interrogator repeats the commandat the same node. This takes advantage of an inherent capability of thedevices, particularly if the devices use backscatter communication,called self-arbitration. Arbitration times can be reduced, and batterylife for the devices can be increased.

When a single reply is read by the interrogator, for example, in node52, the method described in connection with FIG. 4 would involveproceeding to node 53 and then sending another Identify command. Becausea device 12 in a field of devices 12 can override weaker devices, thisembodiment is modified such that the interrogator retries on the samenode 52 after silencing the device 12 that gave the good reply. Thus,after receiving a good reply from node 52, the interrogator remains onnode 52 and reissues the Identify command after silencing the devicethat first responded on node 52. Repeating the Identify command on thesame node often yields other good replies, thus taking advantage of thedevices natural ability to self-arbitrate.

AVALUE and AMASK would have values such as the following for adepth-first traversal in a situation similar to the one described abovein connection with FIG. 4.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 0000 1111* 1000  1111* 1000  1111* 0100 0111 0100  1111* 0100  1111* 1100 1111* 1100  1111*

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

In operation, the interrogator transmits a command at a node, requestingthat devices within the subgroup represented by the node respond. Theinterrogator determines if a collision occurs in response to the commandand, if not, repeats the command at the same node.

In one alternative embodiment, the upper bound of the number of devicesin the field (the maximum possible number of devices that couldcommunicate with the interrogator) is determined, and the tree searchmethod is started at a level 32, 34, 36, 38, or 40 in the tree dependingon the determined upper bound. The level of the search tree on which tostart the tree search is selected based on the determined maximumpossible number of wireless identification devices that couldcommunicate with the interrogator. The tree search is started at a leveldetermined by taking the base two logarithm of the determined maximumpossible number. More particularly, the tree search is started at alevel determined by taking the base two logarithm of the power of twonearest the determined maximum possible number of devices 12. The levelof the tree containing all subgroups of random values is consideredlevel zero, and lower levels are numbered 1, 2, 3, 4, etc.consecutively.

Methods involving determining the upper bound on a set of devices andstarting at a level in the tree depending on the determined upper boundare described in a commonly assigned patent application (attorney docketMI40-118) naming Clifton W. Wood, Jr. as an inventor, titled “Method ofAddressing Messages and Communications System,” filed concurrentlyherewith, and Ser. No. 09/026,043, filed Feb. 19, 1998 and now U.S. Pat.No. 6,118,789, which is incorporated herein by reference.

In one alternative embodiment, a method involving starting at a level inthe tree depending on a determined upper bound (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with a method comprising re-trying on the same node thatgave a good reply, such as the method shown and described in connectionwith FIG. 5.

Another arbitration method that can be employed is referred to as the“Aloha” method. In the Aloha method, every time a device 12 is involvedin a collision, it waits a random period of time before retransmitting.This method can be improved by dividing time into equally sized slotsand forcing transmissions to be aligned with one of these lots. This isreferred to as “slotted Aloha.” In operation, the interrogator asks alldevices 12 in the field to transmit their identification numbers in thenext time slot. If the response is garbled, the interrogator informs thedevices 12 that a collision has occurred, and the slotted Aloha schemeis put into action. This means that each device 12 in the field respondswithin an arbitrary slot determined by a randomly selected value. Inother words, in each successive time slot, the devices 12 decide totransmit their identification number with a certain probability.

The Aloha method is based on a system operated by the University ofHawaii. In 1971, the University of Hawaii began operation of a systemnamed Aloha. A communication satellite was used to interconnect severaluniversity computers by use of a random access protocol. The systemoperates as follows. Users or devices transmit at any time they desire.After transmitting, a user listens for an acknowledgment from thereceiver or interrogator. Transmissions from different users willsometimes overlap in time (collide), causing reception errors in thedata in each of the contending messages. The errors are detected by thereceiver, and the receiver sends a negative acknowledgment to the users.When a negative acknowledgment is received, the messages areretransmitted by the colliding users after a random delay. If thecolliding users attempted to retransmit without the random delay, theywould collide again. If the user does not receive either anacknowledgment or a negative acknowledgment within a certain amount oftime, the user “times out” and retransmits the message.

There is a scheme known as slotted Aloha which improves the Aloha schemeby requiring a small amount of coordination among stations. In theslotted Aloha scheme, a sequence of coordination pulses is broadcast toall stations (devices). As is the case with the pure Aloha scheme,packet lengths are constant. Messages are required to be sent in a slottime between synchronization pulses, and can be started only at thebeginning of a time slot. This reduces the rate of collisions becauseonly messages transmitted in the same slot can interfere with oneanother. The retransmission mode of the pure 11 Aloha scheme is modifiedfor slotted Aloha such that if a negative acknowledgment occurs, thedevice retransmits after a random delay of an integer number of slottimes.

Aloha methods are described in a commonly assigned patent application(attorney docket MI40-089) naming Clifton W. Wood, Jr. as an inventor,titled “Method of Addressing Messages and Communications System,” filedconcurrently herewith, and Ser. No. 09/026,248, filed Feb. 19, 1998, nowU.S. Pat. No. 6,275,476, which is incorporated herein by reference.

In one alternative embodiment, an Aloha method (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with a method involving re-trying on the same node that gavea good reply, such as the method shown and described in connection withFIG. 5.

In another embodiment, levels of the search tree are skipped. Skippinglevels in the tree, after a collision caused by multiple devices 12responding, reduces the number of subsequent collisions without addingsignificantly to the number of no replies. In real-time systems, it isdesirable to have quick arbitration sessions on a set of devices 12whose unique identification numbers are unknown. Level skipping reducesthe number of collisions, both reducing arbitration time and conservingbattery life on a set of devices 12. In one embodiment, every otherlevel is skipped. In alternative embodiments, more than one level isskipped each time.

The trade off that must be considered in determining how many (if any)levels to skip with each decent down the tree is as follows. Skippinglevels reduces the number of collisions, thus saving battery power inthe devices 12. Skipping deeper (skipping more than one level) furtherreduces the number of collisions. The more levels that are skipped, thegreater the reduction in collisions. However, skipping levels results inlonger search times because the number of queries (Identify commands)increases. The more levels that are skipped, the longer the searchtimes. Skipping just one level has an almost negligible effect on searchtime, but drastically reduces the number of collisions. If more than onelevel is skipped, search time increases substantially. Skipping everyother level drastically reduces the number of collisions and savesbattery power without significantly increasing the number of queries.

Level skipping methods are described in a commonly assigned patentapplication (attorney docket MI40-117) naming Clifton W. Wood, Jr. andDon Hush as inventors, titled “Method of Addressing Messages, Method ofEstablishing Wireless Communications, and Communications System,” filedconcurrently herewith, and Ser. No. 09/026,045, filed Feb. 19, 1998, nowU.S. Pat. No. 6,072,801, which is incorporated herein by reference.

In one alternative embodiment, a level skipping method is combined witha method involving re-trying on the same node that gave a good reply,such as the method shown and described in connection with FIG. 5.

In yet another alternative embodiment, any two or more of the methodsdescribed in the commonly assigned, concurrently filed, applicationsmentioned above are combined.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of establishing wireless communications between aninterrogator and wireless identification devices, the method comprisingutilizing a tree search technique to establish communications, withoutcollision, between the interrogator and individual ones of the multiplewireless identification devices, the method including using a searchtree having multiple nodes respectively representing subgroups of themultiple wireless identification devices, the method further comprising,for a node, transmitting a command, using the interrogator, requestingthat devices within the subgroup represented by the node respond,determining with the interrogator if a collision occurred in response tothe command and, if not, repeating the command at the same node.
 2. Amethod in accordance with claim 1 and further comprising, if a collisionoccurred in response to the first mentioned command, sending a commandat a different node, using the interrogator.
 3. A method in accordancewith claim 1 wherein when a subgroup contains both a device that iswithin communications range of the interrogator, and a device that isnot within communications range of the interrogator, the device that isnot within communications range of the interrogator does not respond tothe command.
 4. A method in accordance with claim 1 wherein when asubgroup contains both a device that is within communications range ofthe interrogator, and a device that is not within communications rangeof the interrogator, the device that is within communications range ofthe interrogator responds to the command.
 5. A method in accordance withclaim 1 wherein a device in a subgroup changes between being withincommunications range of the interrogator and not being withincommunications range, over time.
 6. A method in accordance with claim 1wherein the wireless identification device comprises an integratedcircuit including a receiver, a modulator, and a microprocessor incommunication with the receiver and modulator.
 7. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices, the method comprising: establishing forrespective devices unique identification numbers; causing the devices toselect random values, wherein respective devices choose random valuesindependently of random values selected by the other devices;transmitting a communication, from the interrogator, requesting deviceshaving random values within a first specified group of random values torespond; receiving the communication at multiple devices, devicesreceiving the communication respectively determining if the random valuechosen by the device falls within the first specified group and, if so,sending a reply to the interrogator; and determining using theinterrogator if a collision occurred between devices that sent a replyand, if so, creating a second specified group smaller than the firstspecified group; and, if not, again transmitting a communicationrequesting devices having random values within the first specified groupof random values to respond.
 8. A method of addressing messages from aninterrogator to a selected one or more of a number of communicationsdevices in accordance with claim 7 wherein sending a reply to theinterrogator comprises transmitting the unique identification number ofthe device sending the reply.
 9. A method in accordance with claim 7wherein one of the first and second specified groups contains both adevice that is within communications range of the interrogator, and adevice that is not within communications range of the interrogator, andwherein the device that is not within communications range of theinterrogator does not respond to the interrogator.
 10. A method ofaddressing messages from an interrogator to a selected one or more of anumber of communications devices in accordance with claim 7 wherein,after receiving a reply without collision from a device, theinterrogator sends a communication individually addressed to thatdevice.
 11. A method of addressing messages from a transponder to aselected one or more of a number of communications devices, the methodcomprising: establishing unique identification numbers for respectivedevices; causing the devices to select random values, wherein respectivedevices choose random values independently of random values selected bythe other devices; transmitting a communication from the transponderrequesting devices having random values within a specified group of aplurality of possible groups of random values to respond, the pluralityof possible groups being organized in a binary tree defined by aplurality of nodes at respective levels, the specified group beingdefined as being at one of the nodes; receiving the communication atmultiple devices, devices receiving the communication respectivelydetermining if the random value chosen by the device falls within thespecified group and, if so, sending a reply to the transponder; and, ifnot, not sending a reply; and determining using the transponder if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group by descending in the tree; and,if not, transmitting a communication at the same node.
 12. A method ofaddressing messages from a transponder to a selected one or more of anumber of communications devices in accordance with claim 11 whereinestablishing unique identification numbers for respective devicescomprises establishing a predetermined number of bits to be used for theunique identification numbers.
 13. A method of addressing messages froma transponder to a selected one or more of a number of communicationsdevices in accordance with claim 12 and further including establishing apredetermined number of bits to be used for the random values.
 14. Amethod of addressing messages from an interrogator to a selected one ormore of a number of RFID devices, the method comprising: establishingfor respective devices unique identification numbers; causing thedevices to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; transmitting a command using the interrogator requestingdevices having random values within a specified group of a plurality ofpossible groups of random values to respond, the specified group beingequal to or less than the entire set of random values, the plurality ofpossible groups being organized in a binary tree defined by a pluralityof nodes at respective levels; receiving the command at multiple RFIDdevices, RFID devices receiving the command respectively determining iftheir chosen random values fall within the specified group and, only ifso, sending a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting the unique identification number ofthe device sending the reply; determining using the interrogator if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group using a different level of thetree, the interrogator transmitting a command requesting devices havingrandom values within the new specified group of random values torespond; and, if not, the interrogator re-transmitting a commandrequesting devices having random values within the first mentionedspecified group of random values to respond; and if a reply withoutcollision is received from a device, the interrogator subsequentlysending a command individually addressed to that device.
 15. A method ofaddressing messages from an interrogator to a selected one or more of anumber of RFID devices in accordance with claim 14 wherein the firstmentioned specified group contains both a device that is withincommunications range of the interrogator, and a device that is notwithin communications range of the interrogator, and wherein the devicethat is not within communications range of the interrogator does notrespond to the transmitting of the command or the re-transmitting of thecommand.
 16. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 14 wherein the first mentioned specified group contains both adevice that is within communications range of the interrogator, and adevice that is not within communications range of the interrogator, andwherein the device that is within communications range of theinterrogator responds to the transmitting of the command and there-transmitting of the command.
 17. A method of addressing messages froman interrogator to a selected one or more of a number of RFID devices inaccordance with claim 14 wherein a device in the first mentionedspecified group is capable of changing between being withincommunications range of the interrogator and not being withincommunications range of the interrogator over time.
 18. A method ofaddressing messages from an interrogator to a selected one or more of anumber of RFID devices in accordance with claim 14 wherein the devicesrespectively comprise an integrated circuit including a receiver, amodulator, and a microprocessor in communication with the receiver andmodulator.
 19. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 14 and further comprising, after the interrogator transmits acommand requesting devices having random values within the new specifiedgroup of random values to respond; devices receiving the commandrespectively determining if their chosen random values fall within thenew smaller specified group and, if so, sending a reply to theinterrogator.
 20. A method of addressing messages from an interrogatorto a selected one or more of a number of RFID devices in accordance withclaim 19 and further comprising, after the interrogator transmits acommand requesting devices having random values within the new specifiedgroup of random values to respond; determining if a collision occurredbetween devices that sent a reply and, if so, creating a new specifiedgroup and repeating the transmitting of the command requesting deviceshaving random values within a specified group of random values torespond using different specified groups until all of the devicescapable of communicating with the interrogator are identified.
 21. Acommunications system comprising an interrogator, and a plurality ofwireless identification devices configured to communicate with theinterrogator using RF, the interrogator being configured to employ treesearching to attempt to identify individual ones of the multiplewireless identification devices, so as to be able to performcommunications without collision between the interrogator and individualones of the multiple wireless identification devices, the interrogatorbeing configured to follow a search tree, the tree having multiple nodesrespectively representing subgroups of the multiple wirelessidentification devices, the interrogator being configured to transmit acommand at a node, requesting that devices within the subgrouprepresented by the node respond, the interrogator further beingconfigured to determine if a collision occurs in response to the commandand, if not, to repeat the command at the same node.
 22. Acommunications system in accordance with claim 21 wherein theinterrogator is configured to send a command at a different node if acollision occurs in response to the first mentioned command.
 23. Acommunications system in accordance with claim 21 wherein a subgroupcontains both a device that is within communications range of theinterrogator, and a device that is not within communications range ofthe interrogator.
 24. A communications system in accordance with claim21 wherein a subgroup contains both a device that is withincommunications range of the interrogator, and a device that is notwithin communications range of the interrogator, and wherein the devicethat is within communications range of the interrogator responds to thecommand.
 25. A communications system in accordance with claim 21 whereina device in a subgroup is movable relative to the interrogator so as tobe capable of changing between being within communications range of theinterrogator and not being within communications range.
 26. Acommunications system in accordance with claim 21 wherein the wirelessidentification device comprises an integrated circuit including areceiver, a modulator, and a microprocessor in communication with thereceiver and modulator.
 27. A system comprising: an interrogator; anumber of communications devices capable of wireless communications withthe interrogator; means for establishing for respective devices uniqueidentification numbers respectively having the first predeterminednumber of bits; means for causing the devices to select random values,wherein respective devices choose random values independently of randomvalues selected by the other devices; means for causing the interrogatorto transmit a command requesting devices having random values within aspecified group of random values to respond; means for causing devicesreceiving the command to determine if their chosen random values fallwithin the specified group and, if so, to send a reply to theinterrogator; and means for causing the interrogator to determine if acollision occurred between devices that sent a reply and, if so, tocreate a new, smaller, specified group; and, if not, transmit a commandrequesting devices having random values within the same specified groupof random values to respond.
 28. A system in accordance with claim 27wherein sending a reply to the interrogator comprises transmitting theunique identification number of the device sending the reply.
 29. Asystem in accordance with claim 27 wherein a specified group containsboth a device that is within communications range of the interrogator,and a device that is not within communications range of theinterrogator.
 30. A system in accordance with claim 27 wherein theinterrogator further includes means for, after receiving a reply withoutcollision from a device, sending a command individually addressed tothat device.
 31. A system comprising: an interrogator configured tocommunicate to a selected one or more of a number of communicationsdevices; and a plurality of communications devices; the devices beingconfigured to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; the interrogator being configured to transmit a commandrequesting devices having random values within a specified group of aplurality of possible groups of random values to respond, the specifiedgroup being less than the entire set of random values, the plurality ofpossible groups being organized in a binary tree defined by a pluralityof nodes at respective levels, the specified group being defined asbeing at one of the nodes; devices receiving the command beingconfigured to respectively determine if their chosen random values fallwithin the specified group and, only if so, send a reply to theinterrogator, wherein sending a reply to the interrogator comprisestransmitting the unique identification number of the device sending thereply; the interrogator being configured to determine if a collisionoccurred between devices that sent a reply and, if so, create a new,smaller, specified group using a different level of the tree, theinterrogator being configured to transmit a command requesting deviceshaving random values within the new specified group of random values torespond; and, if not, the interrogator being configured to re-transmit acommand requesting devices having random values within the firstmentioned specified group of random values to respond.
 32. A system inaccordance with claim 31 wherein the first mentioned specified groupcontains both a device that is within communications range of theinterrogator, and a device that is not within communications range ofthe interrogator.
 33. A system in accordance with claim 31 wherein adevice in the first mentioned specified group is capable of changingbetween being within communications range of the interrogator and notbeing within communications range of the interrogator over time.
 34. Asystem in accordance with claim 31 wherein the respective devicescomprise an integrated circuit including a receiver, a modulator, and amicroprocessor in communication with the receiver and modulator.
 35. Asystem comprising: an interrogator configured to communicate to aselected one or more of a number of RFID devices; a plurality of RFIDdevices, respective devices being configured to store a uniqueidentification number, respective devices being further configured tostore a random value; the interrogator being configured to transmit acommand requesting devices having random values within a specified groupof a plurality of possible groups of random values to respond, theplurality of possible groups being organized in a binary tree defined bya plurality of nodes at respective levels, the specified group beingdefined as being at one of the nodes; devices receiving the commandrespectively being configured to determine if their chosen random valuesfall within the specified group and, if so, send a reply to theinterrogator; and, if not, not send a reply; and the interrogator beingconfigured to determine if a collision occurred between devices thatsent a reply and, if so, to create a new, smaller, specified group bydescending in the tree; and, if not, to transmit a command at the samenode.
 36. A system in accordance with claim 35 wherein the uniqueidentification numbers for respective devices are stored in digital formand respectively comprise a predetermined number of bits.
 37. A systemin accordance with claim 35 wherein the random values for respectivedevices are stored in digital form and respectively comprise apredetermined number of bits.
 38. A system in accordance with claim 35wherein the interrogator is configured to determine if a collisionoccurred between devices that sent a reply in response to respectiveIdentify commands and, if so, to create further new specified groups andrepeat the transmitting of the command requesting devices having randomvalues within a specified group of random values to respond usingdifferent specified groups until all responding devices capable ofresponding are identified.
 39. A radio frequency identification (RFID)system, comprising: a plurality of RFID tags; and at least oneinterrogator, the interrogator to transmit a first request to theplurality of RFID tags, the first request specifying a first subgroup ofa group of random numbers, among the plurality of RFID tags at least oneRFID tag having generated a random number that is within the firstsubgroup to provide a first response, the interrogator to receive one ormore responses, including the first response, from one or more of theplurality of RFID tags respectively, and the interrogator to repeat thefirst request if the first response is received without a detectedcollision.
 40. The system of claim 39, wherein the first requestincludes a selection indicator; and the one or more of the plurality ofRFID tags provides the one or more responses only if the selectionindicator corresponds to one or more selection bits stored on the one ormore of the plurality of RFID tags.
 41. The system of claim 39, whereinthe one or more of the plurality of RFID tags are configured to set aninventoried flag to a first state to indicate that a respective RFID taghas responded to the interrogator.
 42. The system of claim 39, whereinthe interrogator is configured to transmit a wake-up signal to cause theone or more of the plurality of RFID tags to transition from abattery-saving mode to an operational mode.
 43. The system of claim 39,wherein the interrogator is configured to send a sleep command to theRFID tag that provides the first response.
 44. The system of claim 39,wherein the first response includes the random number generated by theRFID tag that provides the first response; and the interrogator is tosend at least one additional command to the RFID tag, the first RFID tagbeing identified in the at least one additional command by the randomnumber.
 45. A method implemented in a radio frequency identification(RFID) system having at least one RFID tag and at least one RFIDinterrogator, the method comprising: generating random numbers at aplurality of RFID tags, including an RFID tag affixed to an object;transmitting a first command from the interrogator to select RFID tags,including the RFID tag affixed to the object, that have generated randomnumbers that are within a subset of random numbers; the selected RFIDtags transmitting a response, including a random number generated by theRFID tag affixed to the object; if the random number is received at theinterrogator, retransmitting the first command.
 46. The method of claim45, wherein the selected RFID tags transmitting the response to indicatedata stored on the selected RFID tags matches one or more selection bitsspecified by the first command.
 47. The method of claim 45, wherein theRFID tag is further configured to communicate the response at a timebased at least in part upon a random number.
 48. The method of claim 45,further comprising transmitting a command to silence the RFID tagaffixed to the object upon receiving the response.
 49. An interrogator,comprising: one or more antennas to poll a plurality of radio frequencyidentification (RFID) tags; means for transmitting a first command toselect a first subset of the plurality of RFID tags; means for receivingone or more responses from the subset of RFID tags, the one or moreresponses including a random number generated at a first RFID tag of theplurality of RFID tags; means for transmitting the first command if atleast one of the one or more responses is received without a collision;and means for transmitting a second command to select a second subset ofthe plurality of RFID tags if a collision is detected in the one or moreresponses.
 50. The interrogator of claim 49, wherein the first commandincludes a selection indicator that identifies a class of one or more ofa plurality of RFID tags from which a response is being requested. 51.The interrogator of claim 49, further comprising repeatedly redefiningthe subgroup to include fewer possible random numbers and transmitting anew request for RFID tags having a random number within the subgroup torespond until receiving a response without a collision or receiving noresponse.
 52. A method implemented in a radio frequency identification(RFID) device, the method comprising: receiving a first command,comprising a first set of bit values, from an interrogator to select theRFID device based on a comparison between the first set of bit valuesand data stored in the RFID device; transmitting a response to the firstcommand, the response including an identifier of the RFID device; if theresponse is received at an interrogator, remaining silent when the firstcommand is subsequently repeated; and if the response is not received atthe interrogator, retransmitting the response when the first command issubsequently repeated.
 53. The method of claim 52, wherein transmittingthe response is done in a randomly selected slot; and the method furthercomprises transmitting an identification code to identify a person withwhom the RFID device is associated.
 54. A method implemented in a radiofrequency identification (RFID) system having an interrogator to poll aplurality of RFID tags, the method comprising: transmitting a firstcommand to select a first subset of RFID tags; receiving one or moreresponses from the subset of RFID tags, the one or more responsesincluding a random number generated at a first RFID tag of the pluralityof RFID tags; and retransmitting the first command if at least one ofthe one or more responses is received without a collision.
 55. Themethod of claim 54, further comprising: affixing the plurality of RFIDtags to a plurality of objects to track as inventory is added andremoved.
 56. The method of claim 54, further comprising: after receivingthe one or more responses and prior to retransmitting the first command,transmitting a command to silence an RFID tag from which a response isreceived without a collision.
 57. The method of claim 54, wherein thefirst command includes a selection indicator that identifies a class ofone or more of a plurality of RFID tags from which a response is beingrequested.
 58. The method of claim 54, further comprising repeatedlyredefining the subgroup to include fewer possible random numbers andtransmitting a new request for RFID tags having a random number withinthe subgroup to respond until receiving a response without a collisionor receiving no response.
 59. A radio frequency identification (RFID)device, comprising: an antenna; a receiver coupled to the antenna toreceive a first command from an interrogator, the first commandcomprising a first value having multiple bits to select a group of oneor more RFID devices in a field of the interrogator; processingcircuitry to determine if the RFID device is selected by theinterrogator based on the first value; and a transmitter to communicatea response to the first command in a first time slot with a firstprobability if the RFID device is selected by the interrogator inaccordance with the first value, wherein the response includes anidentifier of the RFID device and the first probability is indicated bythe first command; wherein if the first command, including the firstvalue to select the group, is subsequently repeated by the interrogatorwhile the RFID device remains within the field of the interrogator, theRFID device is configured to: remain silent, if the response wasreceived at the interrogator without detecting a collision, andretransmit the response if the response was not received at theinterrogator without detecting a collision.
 60. The RFID device of claim59, wherein the identifier comprises a random number generated by theRFID device.
 61. The RFID device of claim 60, wherein the responseincludes the multiple bits of the first value.
 62. The RFID device ofclaim 59, wherein the receiver is configured to receive signals eachindicating a beginning of a time slot; and wherein the transmitter isconfigured to communicate the response to the first command in aselected one of a number of time slots indicated by the first command.63. The RFID device of claim 59, wherein the response includes themultiple bits of the first value.
 64. The RFID device of claim 59,wherein the RFID device is further configured to transmit anidentification code to identify a person with whom the RFID device isassociated.
 65. The RFID device of claim 59, wherein the interrogator isto transmit the first command before the RFID device first communicatesto the interrogator.
 66. A radio frequency identification (RFID)interrogator, comprising: an antenna; a transmitter coupled to theantenna to transmit a first command comprising a value, having multiplebits, corresponding to a group of one or more RFID devices to beselected in a field of the interrogator; a receiver to receive aresponse to the first command from an RFID device selected by the firstcommand in accordance with the value, the response to include anidentifier of the RFID device and to be received in a time slot inaccordance with a time slot method; and processing circuitry todetermine if the response is received without a collision, and if so, totransmit a second command to silence the RFID device and to retransmitthe first command, including the value, to reselect the group to respondto the first command.
 67. The interrogator of claim 66, wherein theidentifier comprises a random number generated by the RFID device. 68.The interrogator of claim 66, wherein the receiver is to receive theresponse in a first time slot with a first probability in accordancewith the time slot method.
 69. The interrogator of claim 66, wherein thereceiver is to further receive an identification code from the RFIDdevice to identify a person with whom the RFID device is associated. 70.The interrogator of claim 66, wherein the response includes the multiplebits of the value.
 71. The interrogator of claim 66, wherein thetransmitter is to transmit the first command after the RFID deviceenters the field of the interrogator but before the RFID devicecommunicates any responses to the interrogator.
 72. A method comprising:transmitting from an interrogator a first command, including a firstidentifier comprising a plurality of bits, to select a set of one ormore RFID devices, corresponding to the first identifier, in a field ofthe interrogator, and to request the set to respond in accordance with atime slot method in which an RFID device responds in a first time slotwith a probability indicated by the first command; receiving a firstresponse to the first command from a first RFID device of the set,wherein the first response is received in a time slot in accordance withthe time slot method and the first response includes a second identifierof the RFID device; detecting no collision in the first response fromthe first RFID device; and retransmitting from the interrogator thefirst command, including the first identifier, to select the set of oneor more RFID devices and to request the set to respond while the setremains in the field of the interrogator.
 73. The method of claim 72,wherein the second identifier comprises a random number generated by theRFID device.
 74. The method of claim 72, further comprising transmittinga plurality of signals from the interrogator, each of the plurality ofsignals defining the start of a time slot in accordance with the timeslot method.
 75. The method of claim 72, further comprising transmittingfrom the interrogator a second command, wherein the second commandindicates a change in the probability.
 76. The method of claim 72,wherein the response includes the plurality of bits of the firstidentifier.
 77. The method of claim 72, further comprising: receiving anidentification code from the RFID device to identify a person with whomthe RFID device is associated.
 78. The method of claim 72, whereintransmitting the first command is performed after the RFID device entersthe field of the interrogator but before the RFID device communicatesany signals to the interrogator.
 79. A radio frequency identification(RFID) system, comprising: an interrogator to transmit a first commandto select a group of one or more RFID devices in a field of theinterrogator, wherein the first command includes a first identifier,comprising a plurality of bits, that corresponds to the group of one ormore RFID devices; and an RFID device in the field of the interrogatorto determine, using the first identifier, if the RFID device is selectedas one of the group, and if so, to communicate a response to the firstcommand in a first time slot with a first probability, wherein theresponse includes a second identifier of the RFID device; wherein theinterrogator is to retransmit the first command, including the firstidentifier, after the interrogator receives the response, without acollision, from the RFID device and while the group remains in the fieldof the interrogator.
 80. The system of claim 79, wherein theinterrogator is configured to transmit a plurality of signals, eachdefining a start of a time slot for one or more responses to the firstcommand, and the RFID device is configured to transmit the response tothe first command in a randomly selected time slot.
 81. The system ofclaim 79, wherein the response from the first RFID device includes theplurality of bits of the first identifier.
 82. The system of claim 79,wherein the second identifier comprises a random number generated by theRFID device.
 83. The system of claim 79, wherein the RFID device storesan identification code that identifies an associated person.
 84. Thesystem of claim 79, wherein the first probability is indicated by thefirst command.
 85. The system of claim 79, wherein the interrogator isto transmit the first command after the RFID device enters the field ofthe interrogator but before the RFID device subsequently communicates tothe interrogator.